Add PythonPackage base class

- Add a PythonPackage class with build system support.
  - Support build phases in PythonPackage
  - Add a custom sanity check for PythonPackages
  - Get rid of nolink dependencies in python packages

- Update spack create to use new PythonPackage class

- Port most of Python packages to new PythonPackage class

- Conducted a massive install and activate of Python packages.
  - Fixed bugs introduced by install and activate.

- Update API docs on PythonPackage

var/spack/repos/builtin/packages/py-pychecker/package.py

@@ -25,14 +25,9 @@
 from spack import *
 
 
-class PyPychecker(Package):
-    """"""
+class PyPychecker(PythonPackage):
+    """Python source code checking tool."""
     homepage = "http://pychecker.sourceforge.net/"
     url      = "http://sourceforge.net/projects/pychecker/files/pychecker/0.8.19/pychecker-0.8.19.tar.gz"
 
     version('0.8.19', 'c37182863dfb09209d6ba4f38fce9d2b')
-
-    extends('python')
-
-    def install(self, spec, prefix):
-        setup_py('install', '--prefix=%s' % prefix)